When a substance such as human tissue is subjected to an uniform magnetic field (polarizing field Bo), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x, G.sub.y and G.sub.z,) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals (also referred to as MR signals) are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
When viewing an MR image of a structure of interest, such as an anatomical section, the MR imaging system operator may desire to view an MR image in which one or more types of tissue comprising the anatomical section is contrasted with respect to the remaining types of tissue of the anatomical section. Moreover, the operator may desire to modify the image contrast of an MR image acquisition in progress or to prescribe the image contrast prior to an MR image acquisition.
Currently, various image contrast mechanisms such as chemical saturation and spatial saturation are used in MR imaging to generate images of varying contrast. For example, chemical saturation is used to suppress the relatively large magnetization signal from fatty tissue. Each image contrast mechanism is made possible by a corresponding magnetization preparation applied to the anatomical section prior to the MR scan. Briefly, magnetization preparation involves preparing the spin state in the bore such that the anatomical section to be imaged is in a certain magnetized state immediately before the regular image scanning commences. Thus, to acquire an MR image with image contrast, the MR imaging system executes an MR imaging pulse sequence comprised of at least two sets of waveform segments--at least one set of image contrast waveform segment and a set of (regular) imaging waveform segment.
In conventional MR imaging systems, the MR imaging pulse sequence responsible for a specific image contrast mechanism is typically constructed and stored in the MR imaging system prior to scanning. In particular, the MR imaging pulse sequence is comprised of the specific image contrast waveform segment permanently linked to the imaging waveform segment, thus one waveform set. When an operator desires this specific image contrast mechanism, this all-inclusive pulse sequence is evoked and executed in its entirety. The drawback of this type of pulse sequence architecture is that the operator must wait until the image acquisition in progress is completed before new desired image contrast mechanism(s) can be applied. Furthermore, even if the amplitudes of the image contrast waveform segment of the MR imaging pulse sequence can be set to zero while the image acquisition is in progress, essentially prescribing a new image contrast mechanism during acquisition, there is no reduction in acquisition time because the zero amplitude image contrast waveform segment portion must still be executed along with the rest of the MR imaging pulse sequence.